Electrical devices containing conductive polymers

ABSTRACT

Electrical devices which comprise at least one metal electrode and a conductive polymer element in contact therewith, wherein the metal surface which contacts the conductive polymer has a roughened or otherwise treated surface to improve its adhesion to the conductive polymer. The metal electrode is preferably an electrodeposited foil. The conductive polymer preferably exhibits PTC behavior. The devices include heaters and circuit protection devices. The improved adhesion results in improved physical and electrical stability, and broadens the range of conductive polymer compositions which can be used in a number of important applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical devices comprising conductive polymer compositions.

2. Introduction to the Invention

Conductive polymers are well known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer. They can be used in circuits in which current passes through them, eg. in heaters and circuit protection devices, and in such use they may exhibit what is known as PTC (positive temperature coefficient) or ZTC (zero temperature coefficient) behavior. The term "PTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which, in the operating temperature range, has an R₁₄ value of at least 2.5 or an R₁₀₀ value of at least 10, preferably both, and which preferably has an R₃₀ value of at least 6, where R₁₄ is the ratio of the resistivities at the end and the beginning of the 14° C. temperature range showing the greatest increase in resistivity, R₁₀₀ is the ratio of the resistivities at the end and the beginning of the 100° C. temperature range showing the greatest increase in resistivity, and R₃₀ is the ratio of the resistivities at the end and the beginning of the 30° C. temperature range showing the greatest increase in resistivity. The term "ZTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which does not show PTC behavior in the operating temperature range; thus the term is used to include (a) compositions which show no substantial change in resistivity over the operating temperature range (eg. from room temperature to 100° C.), (b) compositions which show substantial increases in resistivity over the operating temperature range but still do not have R₁₄, R₃₀ or R₁₀₀ values as specified above, (c) compositions which show substantial decreases in resistivity over the operating temperature range [often denoted NTC (negative temperature coefficient) compositions], and (d) compositions as defined in (a), (b) and (c) which exhibit PTC behavior at temperatures above the operating temperature range.

Documents describing conductive polymer compositions and devices comprising them include U.S. Pat. Nos. 3,861,029, 4,177,376, 4,237,441, 4,315,237, 4,352,083, 4,413,301, 4,426,339, and 4,459,473, the disclosures of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

The electrodes which have been used to make physical and electrical contact with conductive polymers include solid and stranded wires, metal foils, and expanded and perforated metal sheets. We have now discovered that improved physical and electrical properties can be obtained through the use of electrodes which have a microrough surface which is in direct physical and electrical contact with the conductive polymer. The term "microrough" is used herein to denote a degree of surface irregularity which is different from that observed in stranded wires and perforated metal sheets, and which is sufficient to provide improved physical bonding between the metal and the conductive polymer. Thus in one aspect the present invention provides an electrical device which comprises

(1) an element composed of a conductive polymer, and

(2) at least one metal electrode having a microrough surface which is in direct physical contact with the conductive polymer element.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated in the accompanying drawing in which:

FIG. 1 is a side view of a device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The microrough surface of the electrodes used in the present invention can be prepared in a number of different ways. The preferred method is electrodeposition, the microrough surface being the surface which is exposed to the electrolyte. For example, electrodeposited foils, particularly copper and nickel foils, are preferred for use in this invention. It is also possible to use other processes which result in a similar degree of roughness, eg. irregularities which protrude from the surface by a distance of at least 0.03 microns, preferably at least 0.1 microns, particularly 0.1 to 100 microns, and which have at least one dimension parallel to the surface which is at most 500 microns, preferably at most 100 microns, particularly at most 10 microns, and which is preferably at least 0.03 micron, particularly at least 0.1 micron. The irregularities can be of the same shape as those produced by electrodeposition, eg. generally spherical nodules protruding from the surface, or they can be of a different shape. Such processes can create the microrough surface by removal of material from a smooth surface, eg. by etching, by chemical reaction with a smooth surface, eg. by galvanic deposition, or by deposition of a microrough layer of the same or a different material on a smooth surface. A smooth foil can be treated by contact e.g. rolling or pressing with a patterned surface to generate a microroughness. The microrough surface can if desired be treated to change its chemical characteristics. For example, an electrodeposited metal foil can be passivated i.e. rendered inactive or less chemically reactive, by an appropriate treatment, eg. one which provides a coating thereon of a water-stable oxide, especially a zinc-nickel or nickel treatment of an electrodeposited copper foil. Such treatment is for example desirable where the metal may catalyse degradation of the conductive polymer. Such treatment can also be carried out so as to provide appropriate acid-base interactions with the conductive polymer.

Through the use of microrough surfaces on the electrodes, the range of conductive polymers which can be used is increased. For example, when using a conventional mental foil, it is often necessary for the conductive polymer to include a polar copolymer or other polymeric ingredient which provides improved adhesion to the mental foil but whose presence detracts from the desired electrical characteristics. Thus the invention makes it possible to use a wider range of conductive polymers (both PTC and ZTC) in situations in which separation of the electrode and the conductive polymer is an anticipated problem, either as a result of flexing, different coefficients of expansion, exposure to solvents, eg. diesel fuel, or thermal or electrical shock. Suitable conductive polymers are disclosed in the documents incorporated herein by reference. Preferred conductive polymers include those based on polyolefins, particularly high density polyethylene, and those based on fluoropolymers, particularly polyvinylidene fluoride. Advantages of the improved adhesion include the ability to punch very small parts from foil laminates and substantially improved life, even when exposed to high voltages.

The invention can be used in any of the devices described in the documents incorporated herein by reference, including in particular circuit protection devices, particularly laminar devices having for example a resistance of less than 100 ohms, particularly less than 25 ohms, especially less than 1 ohm. Very small laminar devices, having at least one laminar dimension which is less than 0.2 inch, eg. less than 0.15 inch, and even smaller, such as less than 0.1 inch, can be prepared by punching a foil laminate. Other useful devices include self-limiting heaters, especially flexible sheet heaters having a total surface area of at least 1.0 square inch.

The invention is illustrated by the following Examples.

EXAMPLE 1

The ingredients listed in the Table below were tumble-blended, mixed in a Banbury mixer, melt-extruded into a water bath and chopped into pellets. After drying, the pellets were extruded as a sheet 8.25 inch (21.0 cm) wide and 0.030 inch (0.076 cm) thick, and samples 6 inch (15.3 cm) square were cut from the sheet.

Each sample was laminated between two metal foils 6×6×0.0014 inch in a heated press at 260° C. and 4,000 lb. pressure for 2 minutes, followed by 7,000 lb. pressure for 3 minutes. The metal foils were electrodeposited copper foils which had been passivated with nickel and zinc on the surface adjacent the sample. Such foils are available from Yates Industries under the trade name TEX-1.

Disc-shaped devices 0.125 inch (0.318 cm) in diameter were punched from the laminates. A 24 AWG nickel-plated steel lead was attached to each metal foil on each device. The devices were then encapsulated by an epoxy resin which was cured at 110° C. for 3 hours.

The physical and electrical stability of the foil/conductive polymer interface remained excellent under a wide variety of conditions.

A device 10 of Example I is shown in FIG. 1. The device 10 comprises a laminar element 12 which is composed of a conductive polymer exhibiting PTC behavior and first and second electrodeposited metal foil electrodes (numerals 14 and 16), which electrodes 14 and 16 are in direct physical contact with a surface of the conductive polymer element 12.

EXAMPLE 2

Following substantially the same procedure as in Example 1, devices were made from a conductive polymer containing the ingredients listed in the Table below.

                  TABLE                                                            ______________________________________                                                  Example 1    Example 2                                                                           vol              vol                                Ingredient wt (g)  wt %    %    wt (g)                                                                               wt %  %                                  ______________________________________                                         High density                                                                              6200    31.5    56.1 8092  49.3  63.9                               polyethylene                                                                   (Marlex 6003)                                                                  Carbon black                                                                              5310    27.0    25.7 --    --    --                                 (Sterling SO)                                                                  Carbon black                                                                              --      --      --   8071  49.0  34.1                               (Statex G)                                                                     Titanium dioxide                                                                          7955    40.5    16.5 --    --    --                                 (TiPure R101)                                                                  Antioxidant                                                                                205     1.0     1.7  276   1.7   2.0                               ______________________________________                                          NOTES:                                                                         Marlex 6003 is available from Phillips Petroleum and is a high density         polyethylene with a melt index of 0.3 and a melting point of about             135° C.                                                                 Sterling SO is a carbon black available from Cabot. It has a particle siz      of 41 millimicrons and a surface area of 42 m.sup.2 /g.                        Statex G is available from Columbian Chemicals and is a carbon black with      a particle size of about 60 millimicrons and a surface area of 36 m.sup.2      /g.                                                                            TiPure R101 is a titanium dioxide available from Du Pont.                      The antioxidant used was an oligomer of 4,4thiobis (3methyl-6-tert butyl       phenol) with an average degree of polymerization of 34, as described in        U.S. Pat. No. 3,986,981.                                                 

EXAMPLE 3

The following ingredients were mixed following the above procedure.

    ______________________________________                                                         wt. (g) wt. %   vol. %                                         ______________________________________                                         Carbon black       3,915    19.2    19.0                                       (Vulcan XC-72)                                                                 Polyvinylidene fluoride                                                                          15,701    76.8    77.5                                       (KF 1000)                                                                      Calcium Carbonate   618      3.0     2.0                                       (Omya Bsh)                                                                     Radiation cross-linking agent                                                                      199      1.0     1.5                                       ______________________________________                                    

The pellets were extruded into a sheet 11 inch (27.9 cm) wide and 0.020 inch (0.051 cm) thick and were irradiated 20 Mrad using a 1.5 MeV electron beam. Samples 6 inch (15.3 cm) square were cut from the sheet. Two samples of polymer sheet were laminated between two (6×6 in) sheets of (1 ounce) electrodeposited copper foil by exposing to 1500 lbs pressure at 200° C. for 4 minutes, 20,000 lbs pressure for 2 minutes, before cooling at 20,000 lbs pressure in a water-cooled press. Such foils are available from International Foils. The resulting sheet was 0.035 inch (0.089 cm). Flat 0.005 inch (0.013 cm) copper leads were soldered onto 1 inch (2.54 cm)×2 inch (5.08 cm) samples cut from the slab. The heaters were then encapsulated in epoxy.

KF 1000 is a polyvinylidene fluoride available from Kureha.

Vulcan XC-72 is a carbon black available from Cabot with a particle size of 30 millimicrons and a surface area of 254 m² /g.

Omya Bsh is CaCO₃ available from Omya Inc.

The radiation cross-linking agent used is triallylisocyanurate. 

We claim:
 1. An electrical device which comprises(1) an element composed of a conductive polymer, and (2) at least one metal electrode having a microrough surface which (i) is in direct physical contact with the conductive polymer element, and (ii) has irregularities which protrude from the surface by a distance of 0.1 to 100 microns and have at least one dimension parallel to the surface which is at most 100 microns.
 2. A device according to claim 1 wherein the microrough surface has been formed by an electroplating process.
 3. A device according to claim 1 wherein the electrode comprises an electrodeposited metal foil.
 4. A device according to claim 1 wherein the electrode comprises an electrodeposited copper foil.
 5. A device according to claim 1 wherein the microrough surface consists essentially of nodules having a diameter of 0.1 to 10 microns.
 6. A device according to claim 1 wherein the conductive polymer exhibits PTC behavior.
 7. A device according to claim 6 wherein the polymeric component in the conductive polymer consists essentially of a polyolefin.
 8. A device according to claim 6 wherein the polymeric component in the conductive polymer consists essentially of a fluoropolymer.
 9. An electrical device which comprises(1) a laminar element which is composed of a conductive polymer exhibiting PTC behavior; (2) a first electrodeposited metal foil electrode having a surface which (i) is in direct physical contact with one surface of the conductive polymer element and (ii) consists essentially of nodules having a diameter of 0.1 to 10 microns, and (3) a second electrodeposited metal foil electrode having a surface which (i) is secured to the other surface of the conductive polymer element and (ii) consists essentially of nodules having a diameter of 0.1 to 10 microns.
 10. A device according to claim 9 wherein the polymeric component in the conductive polymer consists essentially of a polyolefin.
 11. A device according to claim 10 wherein the polyolefin is high density polyethylene.
 12. A device according to claim 9 which is a self-limiting heater having a total surface area of at least 1.0 sq. inch.
 13. A device according to claim 9 wherein the polymeric component in the conductive polymer consists essentially of a fluoropolymer. 